When drilling for oil and gas, a drill string is progressively assembled from the surface by consecutively adding segments of drill pipe, while a drill bit at the bottom of the drill string is rotated to form a wellbore. Drilling fluid is pumped downhole through the drill string and up through an annulus surrounding the drill string. A device such as a Rotating Control Devices (RCD) may be used to seal the annulus for closed-annulus drilling operations, such as managed pressure drilling, underbalanced drilling, mud cap drilling, pressurized mud cap drilling, air drilling, and mist drilling. RCDs can also be used as additional safety barriers when drilling conventionally.
RCDs divert drilling fluid (e.g., drilling mud) returning from a well to separators, chokes, and/or other pieces of equipment in a drilling system, rather than up through a flow nipple to a rig floor as in more traditional and common overbalanced drilling. The RCD is in such cases generally mounted above blowout protectors (BOPs) and below the rig floor. The RCD can be installed directly above a drilling annular or in a riser on floating drilling units above or below a tension ring. In some instances, and RCD device is placed in a riser extending between the ocean floor and the surface.
An RCD includes a rotatable sealing element typically carried by a bearing assembly. The sealing element usually comprises an annular elastomeric part (typically of rubber, nitrile, polyurethane, or the like) having an internal diameter sized to seal around the drill pipe and a cage used to provide structural support and to attach to the bearing assembly. The element seals around the drill pipe and is sufficiently compliant to maintain sealing as the drill pipe is rotated and to accommodate a varying diameter of the drill string, such as to pass drill pipe joints, as the drill string is lowered or raised. In some RCDs, the seal rotates with the drill string and in other RCDs the sealing element remains stationary.
As drill pipe is run through the sealing elements and rotated, the elastomers of the elements progressively wear. Rotary seals between rotating and stationary parts of the bearing assembly also wear. Maintenance of the RCD therefore requires regular replacement of these items. The most common method of replacing such annulus sealing assembly components on a wellhead is to remove the entire annulus sealing assembly (with bearing assembly rotary seals and the sealing elements) and replace the worn parts with a redressed bearing assembly carrying fresh sealing elements. This allows the rig to quickly change over from the used annulus sealing assembly to a new one and allows the elements and rotary seals to be replaced and redressed on the used annulus sealing assembly at leisure and with a proper setup of tools, fixtures, lighting, spare parts, and so forth.
During the course of operations, however, drilling mud and cuttings flow around the seals and other closely separated components of the RCD. Over time, material can tend to build up in spaces between separate parts of the annulus sealing assembly and/or the RCD body, thus causing the parts to become seized, cemented, or stuck together. In such cases, use of a pulling tool may sometimes be required to forcibly remove a bearing assembly stuck in the body of the RCD.